[Spike] QtProtobuf: Investigate additonal wellknown type support

https://developers.google.com/protocol-buffers/docs/reference/google.protobuf
Like the Any type there may be additional types in the "well-known" types library which may need additional codegen or features to support it.

 Full list of types that are considered well-known:

google/protobuf/any.proto
google/protobuf/api.proto
google/protobuf/compiler/plugin.proto 
google/protobuf/descriptor.proto
google/protobuf/duration.proto
google/protobuf/empty.proto
google/protobuf/field_mask.proto
google/protobuf/source_context.proto
google/protobuf/struct.proto
google/protobuf/timestamp.proto
google/protobuf/type.proto
google/protobuf/wrappers.proto

Upstream's C++ and Java APIs seem only handle Any type in a special way, providing methods to encode and decode messages. Otherwise those types are only put into a special namespace (C++). There are also utilities for Duration and Timestamp types. Objective-C API is not even handling Any type in a special way.

C# API has special handling for wrapper types, where they are replaced by Nullable<T> where T is the wrapped type.

We may implement the encoding/decoding of Any messages as in C++ API. I don't think that we need any further support for those types as of now.

We may want to provide an easy way to use those files in user programs though so users don't need to bundle all those proto files themselves. That may have consequences for API and ABI stability though. Some well-known types have special JSON representation, so if we ever going to support that it will require some work.